The present invention relates generally to electronic circuits and, more particularly, to a system for canceling distortion caused by electromagnetic coupling among nodes in such circuits.
For millions of homes throughout the world, a convenient way to access a data network such as the Internet is via a data modem connected to a standard, band-limited telephone line. The data modem typically has a transmit amplifier, a receive amplifier and a hybrid which allows full duplex communication of transmit and receive signals across the telephone line.
With the evolution of data networks in general and the Internet in particular, users have begun to demand the speedy transmission of large amounts of data in the form of imagery, music, software downloads and so on. In order to allow data transmission to occur at sufficiently high rates with an acceptable level of accuracy, it is crucial for the transmit and receive amplifiers in the data modem to operate in a highly linear manner. That is to say, the distortion level at the output of each amplifier must be very low compared with the level of the useful signal. In addition, it is important from the customer""s point of view that the data modem be inexpensive and consume little power so as to be of a reasonable size without the risk of overheating.
In an attempt to satisfy these constraints, most full duplex data transmission schemes rely on the use of different frequency bands for the transmit and receive signals such that the transmit and receive amplifiers operate in non-overlapping regions of the frequency spectrum. A xe2x80x9cguard bandxe2x80x9d typically separates the transmit and receive frequency bands. The use of distinct frequency bands helps reduce the amount of leakage from the transmit amplifier through the hybrid into the receive amplifier, which in turn reduces the probability that a symbol output by the receive amplifier will be erroneous.
However, there are strict limitations on the width of a guard band that can be used in a practical system when contemplating the transmission of data at high speeds across a standard telephone channel initially designed for voice transmission in the 300-3500 Hz frequency range. Thus, it is often the case that only a narrow guard band separates the transmit and receive frequency bands. It therefore becomes even more important to make the transmit and receive amplifiers linear so as to reduce the likelihood of the transmit signal being distorted and spilling over into the receive band and also to reduce the likelihood of the receive signal being distorted and spilling over into the transmit band. For this reason, the linearity requirements associated with the transmit and receive amplifiers in a data modem are in some cases so severe as to require a difference of 85 dBc between the levels of signal and distortion at the amplifier output.
Unfortunately, regardless of the degree of linearity of a transmit or receive amplifier, use of the amplifier in a full duplex data modem circuit will nevertheless result in the presence of distortion at the amplifier output. Such distortion can be traced to electromagnetic coupling of signals from other areas of the circuit into the signal present at the amplifier input. The amplifier therefore amplifies both the useful signal and the distortion, resulting in the appearance of an amplified useful signal and an amplified distortion component at the output. The presence of an amplified distortion component at the output makes it appear as though the amplifier did not behave linearly whereas the problem is really rooted in the fact that the useful signal at the input to the amplifier was contaminated with electro-magnetically induced distortion to begin with.
This phenomenon is now described in greater detail with reference to FIG. 1, which shows a transmit amplifier 100 and a receive amplifier 150 assumed to be in proximity to one another in a data modem circuit. The transmit amplifier 100 is implemented as a class AB amplifier with two transistors 110, 120 whose emitters are connected together and also to an input stage 130 of a hybrid. An input voltage VIN(t) is applied simultaneously to the base of both transistors 110, 120. When the input voltage VIN(t) is a sinusoid at a frequency fo as shown in FIG. 2A, the transistors 110, 120 conduct during alternating half-cycles of the sinusoid. Assuming the transmit amplifier 100 to be highly linear from an input-output point of view, an output voltage VOUT(t) taken at the emitter junction and prior to the input stage 130 of the hybrid will very closely resemble the input voltage VIN(t) and will thus be a sinusoid at frequency f0.
The frequency content of the input and output voltages VIN(t), VOUT(t) is shown in FIG. 2B. The frequency spectrum is separated into a transmit band 240, a receive band 230 and a guard band 220 which separates the transmit and receive bands. The input voltage VIN(t) and the matching output voltage VOUT(t) are both represented in the frequency spectrum of FIG. 2B by a spike at a frequency f0 in the transmit band 240, which illustrates the sinusoidal nature of the input and output voltages.
However, as now explained, the currents supplying the two transistors 110, 120 exhibit characteristics which are in contrast to those of the input and output voltages VIN(t), VOUT(t). Because each transistor 110, 120 in the class AB amplifier 100 only conducts during alternating half-cycles, the respective supply currents (denoted IS1(t) and IS2(t)) will also consist of half-cycles. Those skilled in the art will appreciate that such half-cycles are replete with second- and higherorder distortion components.
By way of example and with reference to FIG. 2C, there is shown a trace of the supply current IS1(t) as a function of time along with its associated frequency content in FIG. 2D. Shown at 210 are multiple distortion components that are basically harmonics of the frequency f0. Beat frequencies may also appear due to the introduction of second-, third- and higher-order distortion components by the amplifier 100 in the presence of DMT-type signals. Since the guard band 220 is relatively narrow, significant ones of the distortion components 210 appear in the receive band 230 and, through electro-magnetic induction, these distortion components will affect the current IR(t) and the voltage VT(t) at the input of the receive amplifier 150.
Specifically, the supply current IS1(t) travels around a loop 160 in the data modem circuit which defines a certain surface area. Meanwhile, the input current IR(t) to the receive amplifier 150, as received from an output stage 140 of the hybrid, travels around a different loop 170 in the data modem circuit which will have a surface area of its own. Due to the mutual proximity of the two loops 160, 170, the supply current IS1(t) feeding the transistor 110 in the transmit amplifier 100 will electro-magnetically couple its way into the voltage VT(t) and the current IR(t) at the input of the receive amplifier 150 and will manifest itself as a parasitic distortion component.
This parasitic distortion component depends on the electromagnetic flux induced by loop 160 onto loop 170 (denoted xcfx86160xe2x86x92170). For instance, considering the effects on the voltage VT(t), this can be expressed in mathematical terms as:
VT(t)=VR(t)+VP(t),
where
VP(t)=d(xcfx86160xe2x86x92170)/dt=xcex1xc2x7dlS1(t)/dt=xcex1xc2x7jxcfx89xc2x7IS1(t),
and where j=(xe2x88x921), xcfx89 is the frequency of operation and xcex1 is a coupling factor that depends on the dimensions and configurations of the two loops 160 and 170 and on their relative orientation and proximity. The value of xcex1 is generally unknown a priori and may be complex, meaning that it introduces an arbitrary change in both magnitude and phase.
Thus, it is seen that due to the effects of electromagnetic induction between the two loops 160 and 170, the supply current IS1(t) (containing distortion components in the receive band 230) will couple into the current IR(t), consequently distorting the voltage VT(t) at the input of the receive amplifier 150 regardless of the receive amplifier""s inherent linearity characteristics. A similar effect will arise due to the coupling of the supply current IS2(t) into the current IR(t), which further distorts the voltage VT(t) at the input of the receive amplifier 150.
Those skilled in the art will appreciate that virtually all push-pull transmit amplifier configurations will similarly cause degradations in the performance of a receive amplifier through the effects of electromagnetic induction. Also, such effects are not limited to degrading the performance of the receive amplifier; for instance, the receive amplifier supply current may conversely influence the input current to the transmit amplifier. Generally, any node in the data modem circuit may cause or be affected by this type of electro-magnetically induced distortion.
To combat the above-noted distortion effects, various techniques have been investigated such as reducing the magnitude of the coupling factor a by reducing the areas of the loops 160 and 170 traveled by the currents IS1(t) and IR(t), respectively. Other techniques have included improving the electrical shielding of the various circuit components, enhancing the grounding topology, etc. However, it should be appreciated that the loop areas can never be completely reduced to zero and that improvements to electrical shielding and grounding involve the undesirable consequence of rendering the data modem more expensive, larger and/or more complex.
Thus, it is clear that conventional approaches to reducing electro-magnetically induced distortion in data modems and other circuits are limited in their effectiveness and/or applicability, with the end result being that useful signals contain non-negligible and often unacceptable levels of distortion.
The present invention is directed to the cancellation of distortion caused by electro-magnetic coupling among nodes in an electronic circuit. It is assumed that one or more distortion-causing signal sources can be located in the circuit. To cancel the distortion component of a distorted signal at an affected node, a compensation signal is generated for each distortion-causing signal source. The compensation signal for a particular distortion-causing signal source is generated by first inducing a voltage which is a function of the distortion-causing signal. The induced voltage is applied to one or more cells, generally one for each of the affected nodes.
A cell produces a compensation signal whose magnitude and phase are adjustable through selection of the impedance of various ones of the cell""s impedance elements. The magnitude and phase of the compensation signal can be controlled such that the magnitude is set to equal substantially that of the distortion component and the phase is set to equal substantially the opposite of that of the distortion component. The compensation signal is then combined with the distorted signal at the affected node, thereby canceling the distortion component and leaving behind a xe2x80x9ccleanerxe2x80x9d signal.
Therefore, according to a first broad aspect, the invention may be summarized as a system for canceling a distortion component of a distorted signal. The system includes a coupler positioned in proximity to a conductor carrying a distortion-causing signal, for causing the generation of at least one induced voltage that is a function of the distortion-causing signal, and a cell connected to the coupler, for producing a compensation signal as a function of the at least one induced voltage. The cell includes tunable circuit elements for providing control of the magnitude and phase of the compensation signal.
According to another broad aspect, the invention may be summarized as a system for canceling a distortion component present in multiple distorted signals and arising from electro-magnetic interaction with a distortion-causing signal source. The system includes a coupler positioned in proximity to the distortion-causing signal source, for producing at least one induced voltage that is a function of the distortion-causing signal and, for each distorted signal, a cell connected to the coupler, for producing a corresponding compensation signal as a function of the at least one induced voltage. Again, each cell includes tunable impedance elements for providing control of the magnitude and phase of the corresponding compensation signal.
According to yet another broad aspect, the invention may be summarized as a method of canceling a distortion component of a distorted signal in an electronic circuit. The method includes coupling a distortion-causing signal so as to generate at least one induced voltage that is a function of the distortion-causing signal; generating a compensation signal as a function of the at least one induced voltage and as a function of the impedance of at least one tunable impedance element; and combining the compensation signal with the distorted signal to achieve at least partial distortion cancellation.
The invention may be summarized according to still another broad aspect as a method of canceling a distortion component of a distorted signal in an electronic circuit, including electro-magnetically extracting a portion of a distortion-causing signal to produce two opposite-signed induced voltages that are each a function of the distortion-causing signal; generating a compensation signal as a function of the two induced voltages and as a function of the impedance of at least one tunable impedance element; and varying the impedance of the at least one tunable impedance element until at least partial distortion cancellation is achieved.